This invention relates to optical switches and more particularly to an optical switch involving a photopolymer dispersed within a liquid crystal layer.
As discussed in U.S. Pat. No. 6,111,633 issued to Richard Albert et al there exist liquid crystal based optical switches which do not require polarization techniques in order to switch an incoming light beam from one position to another or in fact to switch it on and off. Optical switches have long been known of the mechanical variety and are used principally in telecommunication transmission systems. The reason for the growth in the development of optical switches centers around the use of optical fibers which can accommodate broad bandwidth signals and hence can convey larger quantities of information. Moreover, light waves used in optical fibers have shorter wavelengths than conventional microwaves and are commonly used in telecommunications systems. As opposed to microwaves, the use of optical fibers and their attendant components result in a dramatic reduction in physical size of the cable plant or switching network. It will be appreciated that optical fibers exhibit little or no electromagnetic or radio frequency radiation and thus result in negligible environmental impact. Moreover, optical fibers are relatively insensitive to electromagnetic and radio frequency interference from the surrounding environment.
While every telecommunications system must include means for switching or redirecting the optical signal, the mechanical switches require relatively high driving power and are subject to wear, abrasion and fatigue. As a result, mechanical switches are prone to failure after repeated use. More importantly, since the optical fibers are relatively small, a small displacement in alignment between the output port fibers or input port fibers can easily cause the mechanical switch to fail. This requires mechanical switches which are expensive due to the small tolerances which are required.
As mentioned in the aforementioned patent, liquid crystal optical switches have been proposed as an alternative to the mechanical variety of optical switches. Typical of such liquid crystal optical switches are those described in U.S. Pat. Nos. 4,792,212; 4,790,633 and 4,813,796 issued to Baker. As mentioned in the aforementioned patent, all of these Baker switches utilize some form of polarization in order to do the switching.
On the other hand, the aforementioned optical switch by Richard Albert et al is one which operates by changing the orientation of the optical axis of the liquid crystal such that the optical axis is either perpendicular to the longitudinal plane of liquid crystal or is aligned parallel to the direction of propagation of the light signal. A light signal meeting the optical axis in an alignment perpendicular to the liquid crystal layer is totally reflected, whereas light traveling parallel to the optical axis passes through the liquid crystal layer. Applying an electric field across the liquid crystal layer changes the orientation of the optical axis from perpendicular to parallel. Thus depending on the application of electrical signal, the light is either reflected or passes through so as to effectuate the switching.
While the Albert et al patent overcomes the necessity for the utilization of polarizing apparatus in polarized light, there appears to be a relatively high insertion loss during the switching process due to the complete reflectivity due to the change in orientation of the optical axis. Moreover, the orientation of the input beam relative to the optical axis is critical in the switches such that if there is any misalignment the switching is deleteriously affected.
Moreover, the switches associated with the Albert et al patent are somewhat larger and more cumbersome than they need to be due to the alignment tolerances which must be maintained.
Rather than utilizing mechanical optical switches, and rather than utilizing either polarization dependent or polarization independent liquid crystal switching devices, in the subject system a photopolymer is dispersed in a liquid crystal used in a cell bounded by transparent electrodes and is patterned through the utilization of interfering laser beams to provide a diffraction grating within the cell. In one embodiment the photopolymer dispersed liquid crystal material is five microns in thickness, with the spatial frequency of the grating depending upon the angle between the two interfering beams, as well as depending upon the frequency of the monochromatic radiation utilized in generating the diffraction pattern. The diffraction pattern is recorded and imprinted into the cell due to the utilization of the photopolymer which serves as an erasable record of the grating, and one which can be overwritten.
It has been found that by the application of an electric signal across the liquid crystal layer, the grating can be made to effectively disappear during the application of the electric field. The response of the switch has been found to be less than five microseconds and the grating erasure is due the fact that when the electrical signal is applied the refractive index of the liquid crystal is matched to that of the photopolymer due to the molecular orientation of the liquid crystal. As a result, there is no diffraction associated and the diffraction grating in essence disappears during the application of the electrical signal.
While the subject invention will be described in terms of diffraction gratings, it will be appreciated that other types of interference fringes can be established by laser-writing holography and computer-generated holography to create a device with tailored properties. Thus, for instance, rather than interfering two plane waves which results in the diffraction grating, the subject invention also includes a plane wave interacted with a divergent spherical wave to result in a positive lens. Again, when the electrical signal is applied to the cell, the light which is ordinarily converged to a certain focus by the interference fringe is now defocused.
The result is a switching device with extremely fast response speed which can be lower than five microseconds, is compact and smaller in size due to the noncriticality of the alignment of the incoming beam and the outgoing beam. It is both cost effective and has a long lifetime due to the fact of there being no moving parts and the inherent long lifetime of a liquid crystal.
The switches are easy to arrange in a matrixed optical switch and the power consumption is in fact quite low. Most importantly since reflection is not employed in the subject switch, insertion losses are less than those devices which employ reflection.
In one embodiment, the cell which is sandwiched between two glass plates and in between two conductive coatings on the plates includes a photopolymer, liquid crystal, a dye, acrylate oligomer and acrylated urothane.
In order to make a polymer dispersed liquid crystal cell, as a first step an ITO glass cell is provided having two glass plates. One glass plate is first coated with itrium tantalum oxide (ITO) on one side of the glass plate. Then a droplet of the polymer dispersed liquid crystal, is placed on the ITO coating. Subsequently, a second ITO coated glass plate, with ITO facing the polymer dispersed liquid crystal is placed over the droplet. The two plates are pressed together to make the polymer dispersed liquid crystal droplet spread symmetrically. In order to control the thickness of the liquid crystal film, a spacer is used to achieve a thickness of about 3-15 microns.
The utilization of the dispersed photopolymer permits the imprinting or recording into the optical switch of the desired grating or infringe pattern. It will be appreciated that the amount by which the incoming light is diverted off axis depends on the spatial frequency of the grating or the spatial frequency of the interference fringe lines. This is readily controllable such that optical switches having cells of differing densities of lines and thus different spatial frequencies may be utilized to provide differing degrees of offsetting of the incoming beam, which, when cascaded together can provide that the output beam be directed to one of two, one of four or one of n discreet locations, thus to convey differing bits of information.
In summary, an optical switch is provided in which a cell comprised of photopolymer dispersed in a liquid crystal sandwiched between two transparent plates is first recorded with a grating utilizing two plane wave laser beams interacting at an angle. This grating is permanently established in the cell such that when the cell is illuminated the incoming beam is diffracted in accordance with the spatial frequency of the grating. When an electric signal is applied across the cell, the refractive index of the liquid crystal matches that of the photopolymer due to the molecular orientation of the liquid crystal and no diffraction occurs because the grating formed in the liquid crystal is temporarily erased or over written. The liquid crystal cell therefore becomes transparent with the application of the electrical signal. Switching occurs by whether or not the grating is present or not, which is in turn dependent upon whether or not there is an electrical signal applied across the cell.